Image sensors are photoelectrical devices that convert optical images into electrical signals using the light-responsive characteristics of semiconductor materials. A typical example of an image sensor is a charge-coupled device (CCD). A CCD image sensor may require external control signals and clocking operations with large voltage swings to obtain acceptable charge transfer efficiency, which may consume a large amount of electric power. Further, a CCD image sensor may require an additional circuit for adjusting image signals and generating standard video outputs. Thus, a CCD image sensor may be disadvantageous for high-density integration. Due in part to such limits on CCD image sensors, a CMOS image sensor has been proposed as an alternative to CCD image sensors. CMOS image sensors may have low power consumption and/or may be suitable for high density integration.
A CMOS image sensor may be suitable for high-density integration and/or may have low power consumption because a CMOS image sensor may be constructed in a relatively simple structure using well-known CMOS technology. A unit pixel of a CMOS image sensor usually includes a photodiode as a light-sensitive element, and one or more field effect transistors for transferring and outputting charges from the photodiode.
A structure of a unit pixel of a CMOS image sensor has been proposed that includes a source follower transistor, in order to improve the noise characteristics of the device. A pixel with a source follower transistor will be explained with reference to drawings as follows.
FIG. 1 is a sectional view showing a pixel of a conventional CMOS image sensor.
Referring to FIG. 1, an N-type photodiode 5 is formed in a P-type semiconductor substrate 1. A transfer gate 3a, a reset gate 3b, and a source follower gate 3c of a corresponding transfer transistor 10a, reset transistor 10b and source follower transistor 10c, respectively, are arranged adjacent the photodiode 5, and are isolated from each other on the semiconductor substrate 1. A gate oxide film 2 is interposed between the gates, 3a, 3b, and 3c, and the semiconductor substrate 1. A floating diffusion region 6 is formed in the semiconductor substrate 1 between the transfer gate 3a and the reset gate 3b. In particular, the transfer gate 3a is disposed on the semiconductor substrate 1 between the photodiode 5 and the floating diffusion region 6. First and second impurity regions 7 and 8 are formed on both sides of the source follower gate 3c in the semiconductor substrate 1. The first impurity region 7 is disposed in the semiconductor substrate 1 between the reset gate 3b and the source follower gate 3c. The floating diffusion region 6, and the first and second impurity regions, 7 and 8, are doped with N-type impurities.
The source follower gate 3c is conductively connected to the floating diffusion region 6 through an upper interconnection (not shown). The floating diffusion region 6 functions as the drain of the reset transistor 10b including the reset gate 3b, and the drain of the source follower transistor 10c including the source follower gate 3c. The first impurity region 7 is supplied with a power source voltage. The second impurity region 8 corresponds to the source of the source follower transistor 10c. 
In operation of the CMOS image sensor with the structure shown in FIG. 1, a voltage is applied to the reset gate 3b and the reset transistor 10b is turned on to supply the power source voltage to the floating diffusion region 6 from the first impurity region 7. Thereafter, the reset transistor 10b is turned off to maintain a potential of the floating diffusion region 6 at the power source voltage level.
When light is incident on the photodiode 5, electron-hole pairs (EHPs) are generated and induce signal charges that accumulate in the photodiode 5. The transfer transistor 10a is turned on, which moves electrons into the floating diffusion region 6 from the photodiode 5. Accordingly, the potential of the floating diffusion region 6 varies, which changes the potential of the source follower gate 3c and the potential of the second impurity region 8 (i.e., the source of the source follower transistor 10c). The potential variation at the second impurity region 8 appears as an electric signal at the output terminal of the device. Thereafter, the reset transistor 10b is turned on again to return the floating diffusion region 6 to the power source voltage. These steps may be repeated to transform optical images into electric signals.
As the integration density in semiconductor devices increases, the transistors of a pixel of an image sensor may be gradually shrunk down in dimensions. As a result, hot carrier effects may occur more frequently in the transistors, which may deteriorate the operational characteristics of the CMOS image sensor. For example, hot carrier effects may increase noise, such as dark currents in the CMOS image sensor, degrading the characteristics of the transistors. In particular, the source follower transistor 10c may be susceptible to hot carrier effects. If signal charges (i.e., electrons) accumulate in the floating diffusion region 6, it may decrease the potential applied to the source follower gate 3c, as well as in the source of the source follower transistor 10c (i.e., the second impurity region 8). As a result, the drain of the source follower transistor 10c (i.e., the first impurity region 7) may remain at the level of the power source voltage. Thus, in the source follower transistor 10c, the drain may become higher than the gate in potential, increasing the potential between the drain and the source. As a result, the source follower transistor 10c may become susceptible to hot carrier effects.
The hot carriers may cause impact ionization around the edge of the drain in the source follower transistor 10c, which may result in the generation of excessive numbers of EHPs therein. Excess minority carriers (holes) generated by the hot carriers may accumulate in the semiconductor substrate I and may raise the potential of the substrate 1. As a result, a forward bias may be formed between the semiconductor substrate 1 and the second impurity region 8, increasing the amount of current between the drain and source of the source follower transistor 10c. As a result, the hot carriers may increase, which may further the deterioration of the operational characteristics of the CMOS image sensor. Further, the excess minority carriers (electrons) generated by the hot carriers may flow into the floating diffusion region 6 or/and the photodiode 5, potentially increasing noise, such as dark currents, therein.